Method of forming a semiconductor device

1. A controller for a power supply comprising:

a control circuit configured to select the controller for operating in a continuous current mode or a discontinuous current mode and to switch therebetween;

a first jitter circuit configured to vary a frequency of a clock signal around a center frequency of the clock signal in response to operating in the continuous current mode; and

a current sense circuit including a second jitter circuit configured to form a DCM jitter signal, the current sense circuit configured to vary a current sense signal by the DCM jitter signal, in response to operating in the discontinuous current mode, wherein the current sense circuit does not substantially vary the current sense signal by the DCM jitter signal in response to operating in the continuous current mode.

2. The controller of claim 1 wherein the controller does not vary the frequency around a center frequency in response to operating in the discontinuous current mode.

3. The controller of claim 1 wherein the second jitter circuit includes a signal variation circuit that forms a value of the DCM jitter signal to vary between a maximum value and a minimum value at a DCM jitter frequency, the second jitter circuit including an amplifier that receives the current sense signal and forms a compensated current signal, in response to operating in the discontinuous current mode, as the current sense signal modulated by the DCM jitter signal.

4. The controller of claim 1 further including a demagnetization detection circuit configured to receive a Demag signal representative of a magnetization state of a transformer and to detect substantially demagnetization of the transformer.

5. The controller of claim 1 wherein the control circuit is configured to select operation in the continuous current mode in response to detecting a combination of substantially no demagnetization of a transformer, and the current sense signal.

6. The controller of claim 1 wherein the control circuit selects operation in the discontinuous current mode in response to detecting a combination of substantially demagnetization of a transformer and the clock signal having substantially the center frequency.

7. The controller of claim 6 wherein the controller counts valleys of a Demag signal representative of a magnetization state of a transformer after detecting substantially demagnetization of the transformer and prior to the clock signal having substantially the center frequency.

8. The controller of claim 1 wherein in response to selecting to switch from the continuous current mode to the discontinuous current mode the controller increases a value of the DCM jitter signal if a first jitter signal of the first jitter circuit was decreasing, or decreases the value of the DCM jitter signal if the first jitter signal was increasing.

9. The controller of claim 1 wherein the control circuit inhibits varying a first jitter signal of the first jitter circuit in response to operating in the discontinuous current mode and inhibits varying the DCM jitter signal in response to operating in the continuous current mode.

10. The controller of claim 1 further including a current sense circuit configured to receive the current sense signal that is representative of a primary current through a transformer and form a compensated current signal;

a switching circuit configured to form a pulse signal responsively to the clock signal having a first frequency, a feedback signal that is representative of an output voltage, and a signal that is representative of the current sense signal; and

a quasi-resonant circuit configured to form a quasi-resonant switching signal responsively to the feedback signal, the compensated current signal, and a detection signal representative of a magnetization state of the transformer.

11. A method of forming a controller for a power supply comprising:

configuring the controller to form a switching drive signal having cycles to control a primary current through a transformer to regulate an output voltage to a desired value, the controller receiving a current sense signal that is representative of the primary current;

configuring the controller to operate in either a discontinuous current mode or to operate in a continuous current mode, and to switch therebetween wherein the controller varies a frequency of the switching drive signal around a center frequency while operating in the continuous current mode and wherein the controller forms a compensated current sense signal that varies a value of the compensated current sense signal around a value of the current sense signal while operating in the discontinuous current mode; and

configuring the controller to transition from the discontinuous current mode to the continuous current mode in response to a discontinuous current mode cycle in which the controller does not does not vary the value of the compensated current sense signal; and

configuring the controller to form a first cycle after transitioning from the discontinuous current mode to the continuous current mode to vary the frequency in a direction that is opposite to a direction in which the controller varied the compensated current sense signal during an immediately preceding discontinuous current mode cycle.

12. The method of claim 11 including forming the controller to not vary the frequency of the switching drive signal around a center frequency while operating in the discontinuous current mode.

13. The method of claim 11 including forming a control circuit to select operation in the discontinuous current mode in response to detecting demagnetization of the transformer and the switching drive signal having substantially the center frequency.

14. The method of claim 11 including forming the controller to vary the compensated current sense signal oppositely to a variation of the frequency of the switching drive signal in response to changing from the continuous current mode to the discontinuous current mode.

15. A method of forming a controller for a power supply comprising:

configuring the controller to form a switching drive signal having cycles to control a primary current through a transformer to regulate an output voltage to a desired value, the controller receiving a current sense signal that is representative of the primary current;

configuring the controller to operate in either a discontinuous current mode or to operate in a continuous current mode, and to switch therebetween wherein the controller varies a frequency of the switching drive signal around a center frequency while operating in the continuous current mode and wherein the controller forms a compensated current sense signal that varies a value of the compensated current sense signal around a value of the current sense signal while operating in the discontinuous current mode; and

configuring the controller to transition from the continuous current mode to the discontinuous current mode in response to a continuous current mode cycle in which the controller does not does not vary the frequency, including forming the controller to vary the compensated current sense signal oppositely to a variation of the frequency of the switching drive signal in response to changing from the continuous current mode to the discontinuous current mode.

16. The method of claim 15 including forming the controller to vary the compensated current sense signal oppositely to a variation of the frequency in response to changing from the continuous current mode to the discontinuous current mode.

17. The method of claim 15 including configuring the controller to form a first cycle after transitioning from the continuous current mode to the discontinuous current mode to vary the compensated current sense signal in a direction that is opposite to a direction in which the controller varied the frequency during an immediately preceding continuous current mode cycle.